This invention relates to magnetic resonance imaging (MRI) systems, and particularly to the radio-frequency (RF) coils used in such systems.
MRI utilizes hydrogen nuclear spins of the water molecules in organic tissue, which are polarized by a strong, uniform, static magnetic field of a magnet (named B0xe2x80x94the main magnetic field in MRI physics). The magnetically polarized nuclear spins generate magnetic moments in the tissue. The magnetic moments point in the direction of the main magnetic field in a steady state, and produce no useful information if they are not disturbed by any excitation.
The generation of a nuclear magnetic resonance (NMR) signal for MRI data acquisition is accomplished by exciting the magnetic moments with a uniform RF magnetic field (named B1xe2x80x94the excitation field). The B1 field is produced in the imaging region of interest by an RF transmit coil which is driven by a computer-controlled RF transmitter with a power amplifier. During excitation, the nuclear spin system absorbs magnetic energy, and it""s magnetic moments precess around the direction of the main magnetic field. After excitation, the precessing magnetic moments will go through a process of free induction decay, releasing their absorbed energy and returning to the steady state. During free induction decay, NMR signals are detected by the use of a receive RF coil, which is placed in the vicinity of the excited volume of the tissue. The NMR signal is the secondary electrical voltage (or current) in the receive RF coil that has been induced by the precessing magnetic moments of the tissue. The receive RF coil can be either the transmit coil itself, or an independent receive-only RF coil. The NMR signal is used for producing MR images by using additional pulsed magnetic gradient fields, which are generated by gradient coils integrated inside the main magnet system. The gradient fields are used to spatially encode the signals and selectively excite a specific volume of the human body. There are usually three sets of gradient coils in a standard MRI system, which generate magnetic fields in the same direction of the main magnetic field, varying linearly in the imaging volume.
In MRI, it is desirable for the excitation and reception to be spatially uniform in the imaging volume for better image uniformity. In a standard MRI system, the best excitation field homogeneity is usually obtained by using a whole-body volume RF coil for transmission. The whole-body transmit coil is the largest RF coil in the system. A large coil, however, produces lower signal-to-noise ratio (SNR or S/N) if it is also used for reception, mainly because of its greater distance from the signal-generating tissue being imaged. Since a high signal-to-noise ratio is desirable in MRI, special-purpose coils are used for reception to enhance the S/N ratio from the tissue volume of interest.
It is desirable for specialty RF coil to have the following functional properties: high S/N ratio, good uniformity, high unloaded quality factor (Q) of the resonance circuit, and high ratio of the unloaded to loaded Q factors. In addition, the coil device may be mechanically designed to facilitate tissue sample (e.g., human body, animal, or other organic tissue) handling and comfort, and to provide a protective barrier between the tissue and the RF electronics. Another way to increase the SNR is by quadrature reception. In this method, NMR signals are detected in two orthogonal directions, which are in the transverse plane or perpendicular to the main magnetic field. The two signals are detected by two independent individual coils which cover the same volume of interest. With quadrature reception, the SNR can be increased by up to 2 over that of the individual linear coils.
In MRI and Magnetic Resonance Angiography (MRA), a neurovascular RF coil is used head, neck/c-spine and vascular imaging without repositioning the sample (e.g., a human patient). The coverage of a neurovascular coil, depending on the usable imaging volume (e.g., a sphere of 45 to 50 cm in diameter), may be about 48 cm (from the top of the head to the aortic arch). It is desirable for the performance, i.e., signal-to-noise ratio (SNR) and image uniformity, of a neurovascular coil to be comparable to a conventional head coil for head imaging and to a stand-alone neck coil for neck/c-spine imaging. For vascular imaging, it is desirable for a neurovascular coil to be able to provide homogeneous images for coverage of the blood vessels from the circle of Willis to the aortic arch for most of the patient population.
To cover the head and neck with a single RF coil, an asymmetric birdcage coil design has been used. In this design, the anterior and posterior parts of a conventional birdcage (Hayes, U.S. Pat. No. 4,692,705) head coil are extended further over the neck and chest regions to cover these regions. The asymmetric birdcage coil is operated in quadrature mode for head and neck imaging.
To further extend the coverage to the aortic arch, a quadrature RF coil has also been implemented by (Misic, et al., U.S. Pat. No. 5,517,120) for neurovascular imaging and spectroscopy of the human anatomy. This neurovascular coil utilizes multiple horizontal conductors and end conductors to distribute the current such that two orthogonal magnetic modes, i.e., one horizontal field and one vertical field, are created by the coil to achieve the quadrature detection of magnetic resonance signal. Mechanically, the neurovascular coil is separated into two shells: an upper shell for the anterior conductors and a lower shell for the posterior conductors. These two shells are connected by a hinge at the middle of the top end of the head coil mechanical housing.
The development of array coil technology (Roemer, et al., U.S. Pat. No. 4,825,162) allows one to image a large field-of-view (FOV) while maintaining the SNR characteristic of a small and conformal coil. Using this concept, a two channel (four linear coils) volume array coil for magnetic resonance angiography of the head and neck has been built. The first channel is a four bar quadrature head coil consisting of two linear coils. Two Helmholtz type coils form the second channel for covering the neck and chest. The two Helmholtz type coils are arranged such that the magnetic fields generated by them are diagonally oriented and perpendicular to each other (i.e., a quadrature coil pair). The quadrature neck coil is attached to the quadrature head coil. Each of the two Helmholtz type neck coils overlaps with the head coil to minimize the inductive coupling between the head and neck coils, i.e., the neck coils are critically coupled to the head coil, to reduce the noise correlation caused by the cross-talk between the head and the neck coils.
A split-top, four channel, birdcage type array coil has also been developed (Srinvasan, et al., U.S. Pat. No. 5,664,568; U.S. Pat. No. 5,602,479) for head, neck and vascular imaging. This split-top head and neck coil consists of a birdcage head coil and two distributed type (flat birdcage type) coils: one for the anterior neck-torso and the other for the posterior neck-torso. The quadrature signal obtained with the head coil is separated into two channel. The anterior and posterior neck-torso coils form the other two channels. The housing of the head and neck coil is divided into two parts: the lower housing for the posterior one half of the head coil and the posterior neck-torso coil and the upper housing for the anterior one half of the head coil and the anterior neck-torso coil. The upper housing is removable, i.e., a split top. The upper housing is secured to the lower housing with a latch during imaging. The inductive coupling between the neck-torso coils and the head coil is minimized by overlapping the neck-torso coils with the head coil.
It is known that significant gains in SNR (about 30%) can be achieved by using two short overlapping decoupled birdcage coils to cover the whole field-of-view compared to a single birdcage coil covering the same field-of-view. Converging the horizontal bars of the short birdcage coil, that covers the top part of the head, to a smaller endring, a further improvement in the SNR (about 40%) and better image homogeneity have been realized.
Employing an asymmetric birdcage head and neck coil enlarges the size of a conventional birdcage head coil to cover the neck region. This compromises the performance (i.e., SNR) of the head-section of the asymmetric birdcage coil as compared to a conventional birdcage head coil. The anterior neck-torso coil section is far away from a patient""s chest (for most of the patient population) and its shape is not optimized to fit the human neck-chest contour. Thus, the performance of the neck-torso section of the asymmetric birdcage coil is lower than that of its head section. The SNR drops quickly from the neck region to the chest region. This limits the coverage of the asymmetric birdcage coil to only the head and neck, not the aortic arch.
The quadrature neurovascular coil design, like the asymmetric birdcage coil design mentioned above, also uses a big single coil for covering the entire FOV from the top of the head to the aortic arch. The anterior chest coil section is also attached to the anterior head coil and far away from a patient""s chest (for most of the patient population). Therefore, this neurovascular coil also has the same weaknesses as those of the above asymmetric birdcage coil, for example: lower SNR for the head imaging as compared to a conventional quadrature head coil and imaging non-uniformity of the chest region due to the quick SNR drop-off in this region.
The coverage of the two channel quadrature (four linear coil elements) volume array coil is only for the head and neck but not for the aortic arch. The quadrature head coil generates magnetic fields in both the horizontal and vertical directions but the quadrature neck coil produces magnetic fields in the diagonal directions. In other words, the B1 fields (the magnetic fields generated by MRI RF coil) of the head region and the B1 fields of the neck region are not quadrature (i.e., not perpendicular to each other). Thus, the each of the two linear neck coil elements has to be critically coupled to the two linear head coil elements simultaneously. This increases the complication of isolating the neck coil from the head coil and makes it less robust for manufacture.
The anterior neck-torso coil of the four channel vascular coil is also attached to the anterior head coil and far away from a patient""s chest (for most of the patient population). In addition, both the anterior and posterior neck-torso coils are linear coils. These result in insufficient sensitivity/penetration in imaging the chest region and therefore, cause substantial image non-uniformity for vascular imaging from the circle of Willis to the aortic arch. Image intensity correction is needed to improve the image homogeneity. The decoupling of the multiple modes (i.e., multiple NMR frequencies) birdcage type anterior and posterior neck-torso coils from the multiple modes birdcage head coil is much more difficult than that between two single mode linear coils. The big split-top housing, when being moved toward a patient""s face, may cause some patients to feel threatened.
The major disadvantages of the above designs are 1) lower SNR for head imaging as compared to a conventional quadrature head coil, 2) for large field-of-view imaging, i.e., from the top of the head to the aortic arch, the image uniformity is not good due to rapid signal drop-off at the chest region and 3) lack of a capability of being used as a neck-only/c-spine-only coil.
The optimized birdcage array coils can only cover the head and part of the neck but not the aortic arch. Furthermore, the multiple modes birdcage coil design makes it more difficult to decouple the array coils from each other. This reduces the flexibility of adding more coil elements to the birdcage array coils to extend its coverage to the aortic arch.
An MRI array coil system for neurovascular and spine imaging of a human includes a neck coil having a split top; a dome-like head coil having a dome region, the head coil being slidable between a closed position adjacent to the neck coil and an open position spaced away from the neck coil; a posterior torso coil attached to the neck coil; and an anterior torso coil adapted to cooperate with the posterior coil.